Gallium nitride (GaN) has excellent characteristics in comparison with silicon (Si). For example, the former has a band gap approximately three times that of the latter, a high breakdown electric field intensity approximately 10 times that of the latter, and larger saturation electron velocity, and the like. GaN is expected to simultaneously establish both high withstand voltage and small loss, i.e., low on-resistance, that conventional Si power devices can hardly achieve. GaN is thus expected to be applied to power devices (power semiconductor devices).
Conventionally, there have been proposed Schottky barrier diodes (SBDs), pn junction diodes, metal-insulator-semiconductor (MIS) transistors and other semiconductor devices that employ a GaN substrate (see Japanese Patent Laying-Open No. 2006-100801 (Patent Document 1) for example). Conventionally, power devices have employed a material of GaN generally in the form of a GaN epitaxial layer deposited on a substrate of sapphire, silicon carbide (SiC) or the like, i.e., different in composition. In contrast, a GaN epitaxial layer deposited on a GaN substrate has a lower impurity concentration and a lower dislocation density than a GaN epitaxial layer deposited on a substrate different in composition. Accordingly it is disclosed that epitaxial growth of GaN on a GaN substrate can implement a high withstand voltage and low on-resistance power device (see Tatsuya TANABE et al, “Epitaxial Growth of GaN on GaN Substrate and Its Application to Power Device”, SEI Technical Review, No. 170, (Non Patent Document 1) for example).
Furthermore, a field plate (FP) structure is disclosed as a structure for reducing or preventing an electric field that is concentrated at an end portion of an electrode of a power device to achieve high withstand voltage (see Yoshiharu TAKADA et al, “AlGaN/GaN HEMT power device”, Toshiba Review, Vol. 59, No. 7 (Non Patent Document 2) for example).    Patent Document 1: Japanese Patent Laying-Open No. 2006-100801    Non Patent Document 1: Tatsuya TANABE et al, “Epitaxial Growth of GaN on GaN Substrate and Its Application to Power Device”, SEI Technical Review, No. 170, January 2007, pp. 34-39    Non Patent Document 2: Yoshiharu TAKADA et al, “AlGaN/GaN HEMT power device”, Toshiba Review, Vol. 59, No. 7, July 2004, pp. 35-38